This invention relates to a method of freezing foods and, more particularly, to a freezing method for long time preservation of meats, sea foods, vegetables, and/or other perishable foods as in their fresh condition.
For freezing articles of food it has been generally known to employ a liquid immersion method in which the article is immersed in a brine (or an anti-freezing solution) cooled to a very low temperature, or a liquefied gas method in which a liquefied gas having a low boiling point (such as liquid nitrogen, liquid carbon dioxide, or the like) is sprayed over the article, or in which the article is dipped into the liquefied gas. While conventionally employed in freezing perishable foods, such as meat, fish, and vegetables, these methods have common disadvantages as pointed out below.
In either the liquid immersion method or the liquefied gas method, the entire surface of the article being frozen is cooled to a very low temperature in a short period of time, so that freezing takes place first with the entire surface of the article. This involves the difficulty that once the surface is solidly frozen, an upward volume change which would otherwise result from the freezing of water content of the article during the subsequent interior freezing of the article is prevented, an internal pressure being thus produced within the article. This internal pressure will increase as freezing progresses, and thus may eventually cause cracking to the exterior of the frozen article or lead to disruption of the tissue of the frozen article. Such cracking or tissue disruption may be a cause of dripping when the frozen article is thawed out, thus resulting in a loss of weight and/or deterioration in quality characteristics such as taste and flavor.
Another difficulty is that the brine or liquefied gas in a tank is higher in temperature adjacent its surface at which it is in contact with the atmosphere than at a deeper level, there being a temperature gradient in the height-wise direction of the tank. As such, the freezing conditions vary according to the depth of immersion each individual article to be frozen, which may result in variation in quality among frozen articles. In order to overcome this difficulty, the usual practice is to constantly agitate the brine or liquefied gas in the tank. However, it is impracticable to completely eliminate such temperature gradient or unevenness, since the lower the temperature of such liquid, the greater is its viscosity (which is true more particularly with brine).
Furthermore, aforesaid methods are costly and uneconomical, because they require brine and liquefied gas respectively, both rather expensive, in a large amount.